Relay control system



Dec. 28, 1948. G. G. ROBERTS 2,457,730

RELAY 'cbn'TRoL'sYs'rEM Fi ed Au 29, 1942 I Q 2 Sheets-Sheet 1 INVENTORGerald G- Roberts BY M102;

ATTORNEYS Dec. 28, 1948. ROBERTS 2,457,730

RELAY CONTROL SYSTEM Fi 'ed Aug. 29, 1942 2 Sheets-Sheet 2 F E INVENTORGerald G. Roberts ATTORN EYS Patented Dec. 28, 1948 UNITED STATES PATENTOFFICE RELAY CONTROL SYSTEM Gerald G, RObertmAkromOhin, assignor toWilliam ONeil, Akron, Ohio;

Application August 29, 1942; Serial No..456,'630

7 Claims. 1

This application is a continuation-in-part of my application Serial No.415,398, filed October 17, 1941 Patent No. 2,330,241, granted September28; 1943. The invention relates to a communication system and moreparticularly to a radio control device which may be remotely operated inresponse to transmitted radio frequenciesto perform various functions orto insure the reception of special messages.

It is frequently desirable, especially in modern warfare, to havedevices capable of being operated from a central control station. Thusin tanks which are deployed in the field of battle it is desirable togive a plurality of visual or audible signals from a central station towarn the operator or to cause the operator to change directions, etc. Inorder tohave a control devicecapable of performing a large number offunctions, it'is almost necessary that the individual control elementswhich operate the separate functions be each responsive to a definitefrequenc or at least to a very limited band of frequencies. It is almostmandatory if the device to be controlled is subject to appreciablevibration that the control devices be free of mechanicallytuned'apparatus which might be affected by jars or mechanical vibrationsof certain frequencies.

Remotely operative radio control devices have heretofore been proposedto operate switches'and perform various functions. The control devicesheretofore proposed, however, have been objectionable for the reasonthat they are responsive to too wide a band of frequencies and for thereason that they are adversely afiected by vibration so that they cannotbe used for controls'oi tanks or for giving signals where substantialvibration is experienced.

It is an object of the present invention to provide a radio controldevice which is responsive to definite modulations of radio frequenciesto remotely operate mechanical elements or'to insure reception ofspecial messages and the like.

It is another object of the present invention to provide a radio controldevice responsive'to definite modulations of radio waves and which isnotaffected by jarring, so that it may be used in tanks, aircraft and thelike.

It is another object of the present invention to provide a radio controldevice having a plurality of control elements, each responsive todefinite modulations or a small band of modulations in carrier frequencyto accomplish a given result.

It is a further object of the present invention to provide a radiocontrol device which when connected to the output of a suitableradioreceiving 2" set wllloper-ate'in response to a particular frequency orvery small band of frequencies carried as modulations on the radio-wavesto-which the radio receiver is tuned to operate a switch or givencontrol'mechanism, but which is'notresponsive to frequencies outside ofsaid band.

It is astill further object of the present invention to provide apluralit of'radio control devices which may be connectedto-the'audiooutput of a' radioreceiver, each of which is onlyresponsive-toa definite narrow'band of modulations in the radio wave" towhich said receiver is tuned cooperate-a givencontrol-device or toperforma given-function.

Other objects will be apparent from-thefollowingdescription of thedrawing, in which:

Figure 1 is a schematic diagram ofa control device embodying the presentinvention and'employing two circuit portions; and

Figure 2 is a schematic diagram-of a control device employing a greaternumber of circuit por tions to permit'functional operations responsiveto more than two control frequencies.

Control or signal devices embodying thepresent inventioncomprise'essentially a plurality of circuit portions, each portionhavingconnecting meansfor connection to asuitable power supply;

a grid-controlledelectronic tube connected to said connecting meansandadapted to pass plate current from said power supply or to substantiallyprevent current fljow, depending on the potential appliedto saidggrid; arelay operatively'controlled by a change in said plate current;separating meansfor separating an impedance potential due to signalsofone of a plurality of control frequenciesya rectifier for saidseparating means for obtaining arectified" current due to said impedancepotentials causedby said control frequencies; and meansforimpressing'a'potential dropdue to said rectifiedcurrent on the grid ofan electronic-tube in another of said circuit portions, so that when acontrol signal'within one particular narrow band offrequencies is passedinto said signal device a potential due to the'signalis separated andanegative potential is applied to the grid of an electronic tube in oneof said circuit portions, and when another control signal within anothernarrow wave-band is passed into said signal device a negativepotentialis applied to the grid of, an electronic tube in-the other of saidcircuit portions. The plate current in each of said circuit portionsmaytherefore be controlled by transmission of separate controlfrequencies to said. device;

Preferably, each circuit portion of the control device of the presentinvention-also-comprises an amplifier for increasing the separatedpotential due to each control frequency, means connected into the platecircuit; of said amplifier for separating the amplified potential due tosaid control frequency, means for impressing the amplified potentialthus obtained on said rectifier and means for smoothing the wave-form orfor delaying the rectified current.

Referring to the drawings, in which like parts are indicated by likenumerals of reference throughout the several views, and moreparticularly to Figure 1, the control or signal device embodying thepresent invention may be suitably connected to the audio output of asuitable radio receiver I, such for example as a three-tube receiverhaving a mixer tube, one stage of I. F. diode-detector and a high mu,high impedance audio tube. The output or audio tube of said receiver mayfor example be designated as T with a plate 2. The negative terminal ofthe power supply as Well as of the cathode of the tube T may begrounded. The positive terminal of the power supply is designated as B+.

. The control or signal device of the present invention is responsive toat least two control frequencies to cause at least two functionaloperations. The two circuit portions are designated generally as A andB; each circuit portion has suitable connecting means, such as wires 3aand 3b, respectively, to power supply B+. Suitable grid-controlledelectronic tubes Va and Vb of the respective portions A and B aresuitably connected through said connecting means to said power supplyand are adapted to pass plate current from said power supply or tosubstantially prevent plate current, depending upon the potentialapplied to grid 1 thereof. The relays R9. and Rh are operativelycontrolled by the electronic tube of the respective circuit portion orby the plate current thereof.

For each circuit a separating means such as a tuned circuit or network4a and 4b is provided to obtain a potential drop due to one of saidcontrol frequencies. The circuit constants as determined by theinductances and capacitors L1 and C1 of the circuit 4a and L2 and C2 ofthe circuit 41) are difierent so that the networks 4a and 4b are tunedto resonance for different control frequencies F1 and F2, which may betransmitted as modulations of theradio frequency to which the receiver Iis tuned. The networks 4a and 4b are connected through respectiveisolating resistors 5a and 5b to a coupling condenser 6 which makesconnection to the plate 2 of the tube T, or to other suitable supply orline over which control signals are to be sent, and permits passage ofonly alternating current or the variable portion of the output or platecurrent of the receiver I. An amplifier, to amplify any potential dropacross the network due to the presence of a control current of frequencycorresponding substantially to that to which the particular circuit 4aor 4?), etc. is tuned, especially when used with an additional filteringor tuned circuit to separate amplified potential, markedly increases thesensitivity or selectivity of the device. In the circuit shown, thetubes Va and Vb are of the remote cut-off pentode and duodiode type;such tubes are desirable in that they may perform the functions of thegrid-controlled electronic tube, as well as those of the amplifier andrectifier. Thus, when the grid potential is sufficiently positive topermit passage of plate current, the tubes Va. and Vb may serve asamplifiers to amplify the voltage drop across the respective networks toand 4b, respectively,

4 due to a control frequency to which the particular network 4a or 41)is tuned. Means such as a coupling condenser 8 or 811 and suitableconnector are therefore provided for suitably connecting a positiveportion of the respective network 4a or 4b to the grid 1 of theappropriate tube Va or Vb.

A second tuned network 9a and 911, having respective inductances La andL4 and respective capacitors C3 and C4 in the circuit portions A and B,so selected that its closed circuit is tuned to the same controlfrequency F as the network 4a or 41) of the same circuit portion, isconnected into the plate circuit of the tube Va or Vb of each of thecircuit portions A or B, respectively. The network 9a or 9b togetherwith the condenser Illa or [0b serves as a means for separating the D.C. plate current of the tubes Va or Vb from the amplified potential dropdue to amplified current of the control frequencies F1 or F2 to whichthe circuit 4a or M; of the respective circuit D01- tion is tuned.Connecting means, including the wires [2a and i217, which arerespectively connected to the more negative terminal of the respectivetuned circuits 9a and 5b, are provided for applying the voltage dropacross the circuit 9a or Eib to the respective terminals l3a and |3b ofthe diode rectifier in the appropriate tube Va or Vb. The more positiveside of the networks 9a and ill) are preferably connected to therespective accelerating grids Ma and Mb. The diode rectifier serves asmeans for rectifying the current due to the control frequency.

Filtering or delaying means, comprising suitably arranged resistors [5aor [51) and l'la or llb together with a condenser 18a or I8b, ispreferably connected to the diode rectifier. The filtering means serveto smooth out the wave-form of the rectified current due to amplifiedpotential drop caused by the control frequencies and to provide thedesired delay, so that the control device is not responsive to controlsignals unless they are transmitted for a reasonable period of time.Means such as a wire 20a or 202) and a resistor Zia, or Zlb are providedfor connecting the rectifier or said filtering means to the grid of anelectronic tube in a cooperating circuit portion- Thus, when only twocircuit portions are used, a wire Ella and a resistor 2Ia makeconnection between the grid 1 of the tube Vb of circuit portion B andthe ungrounded side of the condenser lSa in the circuit portion A,whereas the resistor Nb and the wire 20?) serve as means for connectingthe ungrounded side of the condenser 18b to the grid of the electronictube in the circuit portion A.

The respective coils 23a and 23b of the relays Ra and Rh, preferablywith a by-pass resistor 24a or Mb, may be connected in series with theplate lie or 25b of the electronic tube of the respective circuitportion, so that plate current from said tube passes through the coils23a or 2327 to operate the respective relay.

In the operation of the devices of the present invention, signals ormodulations transmitted by the station to which the radio receiver l istuned pass from the output thereof through the coupling condenser 6through the isolating resistors 5a and 5b to the networks 4a and 4b. Ifthe frequency of the signal received corresponds substantially tofrequency F1, to which the circuit 4a is tuned, circuit 411 operates asa high impedance and a substantial potential drop is produced betweenterminal l25a and the ground. If the freqency of the signal receiveddiffers substantially from the frequency F1, the circuit 4a allowsittopass to ground without appreciable potential drop;

The potential due to the frequency F1 is applied through the condenser 8to the grid 1 of the tube Va, It appears in amplified form across thenetwork 9a, also tuned to frequency F1 and connected in the platecircuit of thetube Va. The potential drop across the network 9a isapplied through the coupling condenser Ma to the diode rectifier I3a.

The negative direct current voltage developed at the diode is delayedand its Wave-form (smoothed by the resistances l5a and I'la,togetherwith the condenser l8a. It is applied to the grid "of the tube Vb in thecooperating circuit portion B. This negative voltage causes the normalplate current of the tube Vb to cease, de-energizing the relay coil 23band causing operation or movement of the relay Rb so that contact ismade between contacts and 3|. Closure of the contacts '33 and 3| permitscurrent to flow from the power supply P along wires '32 between contacts33 and 34 of the relay Ra along wires 35 and 36 through a coi1 3! of therelay R, causing it to close and cause one of the functional operationsof the control device. The relay 'R may,for example, operate a pluralityof switches and preferably has holding means, such as contacts 38 and 39which are connected in parallel with the contacts 30 and 3| of therelayRb for maintaining closure, so that upon closure of the relay R itis held closed regardless of the position of the contacts 30 and 3|.

It is seen that except for the short interval of time required to causeinitial operation of relay Rb, transmission of the control frequency F1,to which network 4a is tuned, may be discontinued or it may betransmitted at suitable periodic intervals to cause periodic operationof relay Rb without interfering with the connections initiallyestablished. Thus F1 is available and may be used for sending of codemessages or with the aid of the flapper of the relay Rb and a suitablyattached ratchet switch to accomplish additional functional operations.

If a signal is received of frequency corresponding to frequency F2, towhich the network 4b is tuned, a substantial potential drop is obtainedbetween the terminal I25?) and the ground. This potential drop isamplified, rectified and delayed similarly to the potential drop due tothe frequency F1, and after passing through the delay network,comprising the resistors 15b and Nb and the condenser 3b, is applied asa negative voltage to the grid 7 of the tube Va, causing cessation ofplate current therein and de-energization of the coil 23a of the relayRa, breaking contact between contacts 33 and 34, and causingdeenergization of the coil 3! of the relay R and causing opening of thecircuits closed by the previous operation of the relay R.

As shown in the drawing, the operation of the relay B may be utilized toprovide any desired number of functions; thus, closure of contacts atand. M permits power to be obtained between terminals 52 and 43, whichmay be respectively connected by wires I I0 and III to any desiredapparatus lo]. Closure of contacts 44 and 43 permits power to beobtained between terminals 46 and 4?, which may be respectivelyconnected by wires H2 and H3 to any desired apparatus Hi0. Contacts 48and 49 are a power switch in parallel with that of a standard receiver,such as that schematicallydesignated by numeral 30 and having two tubesI02 and I33,- as shown.

A contact 61 may :be connected to the input or grid of a suitable audiostage of the receiver 5.. a contact '52 may be connected to the outputof anintermediate tube of the standard receiver, and a contact 53 may besuitably connected 'to the output of the auxiliary or midget receiver I,so that upon energization of the coil of the relay R connection isbroken between the contacts 54 and 52, disconnecting the radio frequencystages from the audio of the receiver, and made between contacts 5| and53 connecting the output of the midget receiver to the audio stages ofthe standard receiver 50. It is thus 'seen' that in response to a signalof a frequency F2 with the arrangement shown, the broadcast to which themidget receive-r may be fixedly tuned will be immediately heard throughthe standard receiver 50, even though the latter is tuned to a stationother than that to which the midget receiver'is fixedly tuned. Inresponse to a signal of frequency F2, the standard receiver is againconnected to its normal radio frequency stage.

The modification shown in Fig. 2 illustrates the method of connectingadditional circuit portions, such as circuit portion C, so that therelay of each portion is operatively controlled by a given controlfrequency Or by transmission of modulations of a given control frequencyin a radio carrier wave to which the radio receiver I is tuned. Theconnections to the contacts of relays Ra, Rb and Re of the respectivecircuit portions A, B and C may be arranged in any desired manner,depending on the functional operation desired. It will be seen that amore negative portion of the rectifiers in each of the circuit portionsis connected to the control grid of only one electronic tube in anothercircuit portion. Thus, the rectifier l3a of the circuit portion A isconnected in series through the delay network comprisin the condenser laand resistors l5a and Ila, the resistor Ma and the wire 20a to the grid1c of electronic tube Vc; the rectifier l3b of the circuit portion B isconnected in series through the delay network comprising the condenserlb and the resistors l5b and llb, the resistor 2lb and the wire 20b tothe grid 1 or the tube Va; and the rectifier 130 of circuit portion C isconnected in series through the delay network comprising the condenser Iand the resistors I50 and He; the resistor Ho and the wire 200 to grid 1of the electronic tube Va. Thus the relay Re. is responsive to frequencyF2; relay Rb is responsive to frequency F3 to which circuit 4c is tuned;and relay Re is responsive to frequency F1.

While I have shown control devices having only two and three circuitportions inthe drawin'gs, it is obvious that the number of circuitportions may be increased as desired to accomplish any number offunctional operations. A single circuit portion in a single cut-offfrequency may be used to stop current flow in all desired circuitportion-s.

Control devices of the present invention may have exceedingly highsensitivity and very high selectivity. With the use of an amplifier andtwo networks tuned to the same frequency in each circuit portion, asshown in the drawing, it is found that the controls to accomplish agiven function are sensitive to signals in a wave-band having a widthmuch less than 10 per cent of the signalfrequency and insenitive tosignals outside this'bandy With the sharp cut-of! obtainable by thepresent invention, alarge number of signals to accomplish differentfunctions maybe grouped relatively close together.

, It is to be understood that variations and modifications of thespecific devices herein shown and described for purposes of illustrationmay be made without departing from the spirit Of the invention.

What I claim is: 1. A control or signal device adapted to be connectedto an audio output portion of a radio recelver and responsive to aplurality of control frequencies to cause a plurality of functionalOperations, comprising a plurality of circuit portions, each portionhaving connecting means for connection to a suitable power supply, agridcontrolled electronic tube connected to said connecting means andadapted to pass plate current from said power supply or to substantiallyprevent current flow depending on the potential applied to the gridthereof, a relay operatively controlled by a change in said platecurrent, a filter means for selectively obtaining a higher potential atone of said control frequencies than for others, an amplifier connectedto said filter means for increasing the potential at said controlfrequency, second filter means connected to the output circuit of saidamplifier for separating the amplified potental at said controlfrequency, a rectifier connected to said second filter means forobtaining a rectified potential due to said amplified potential, meansfor delaying said rectie fled potential and means for impressin saidrectified potential on the grid of an electronic tube in one of theother of said circuit portion-s, so that when said amplified potentialin one of said circuit portions is of substantial magnitude 9. negativepotential due to said rectified current is applied to said grid inanother of said circuit portions, said filter means in one of saidcircuit portions being adapted to selectively obtain a potential at oneof said control frequencies and the filter means in other Of saidcircuit portions being adapted to selectively obtain a potential atanother of said control frequencies, so that in response to one controlfrequency flow of plate current in one of said circuit portions is sub--stantially prevented and in response to another control frequency thefiow of plate current in the other of said circuit portions issubstantially prevented, whereby said relays in said circuit portionsare selectively operated in response to said control frequencies.

2. A control or signal device adapted to be connected to an audio outputportion of a radio receiver and responsive to a plurality of controlfrequencies to cause a plurality of functional operations, comprising aplurality of circuit portions, each portion having connecting means forconnection to a suitable power supply, a grid-controlled electronic tubeconnected to said connecting means and adapted to pass plate currentfrom said power supply or to substantially prevent current flow,depending on the potential applied to the grid thereof, a relayoperatively controlled by a change in said plate current, a circuittuned to one of said control frequencies and connected through aresistance to an output portion of said receiver to obtain a potentialdrop at one of said control frequencies, an amplifier tube to amplifysaid potential drop, means for suitably connecting said tuned circuit tothe grid of said amplifier tube, a second circuit tuned to substantiallythe same control frequency connected in series with the plate supply andthe plate of said amplifier tube to obtain an amplifiedfpotential dropdue to said amplified control fredue to said amplified potential drop",means for suitably connecting said second tuned circuit to saidrectifier, means for smoothing out the waveform of said rectifiedcurrent, and means for impressing a negative potential due to saidrectified current on a control grid of said electronic tube in anotherof said circuit portions, said tuned circuits in different circuitportions being tuned to different control frequencies, so that inresponse to one control frequency flow of plate current in one of saidcircuit portions is substantially prevented and in response to anothercontrol frequency the flow of plate current in the other of said circuitportions is substantially prevented, whereby said relays in said circuitportions are selectively operated in response to said controlfrequencies.

3. In a control or signal device responsive to at least two controlfrequencies to cause at least two functional operations, two circuitportions, each portion having a source of power supply, agrid-controlled electronic tube adapted to pass plate current from saidpower supply or to substantially prevent current flow, depending on thepotential applied to the grid thereof, a relay operatively controlled bysaid current, filter means for obtaining a potential drop at one of saidcontrol frequencies, an amplifier tube connected to said filter meansfor amplifying said potential, :1 rectifier connected to the output ofsaid amplifier tube for obtaining a rectified potential corresponding tosaid amplified potential, means connected to said rectifier forfiltering and delaying said rectified potential, and means forimpressing a more negative portion of said rectified potential on acontrol grid of said electronic tube in another of said circuitportions, the filter means in each of said circuit portions beingadapted to separate potential due to a different control frequency, sothat in response to one control frequency fiow of the plate current inone of said circuit portions is substantially prevented and in responseto another control frequency the flow of plate current in the other ofsaid circuit portions is substantially prevented, whereby the relay ofone of said circuit portions is operated in response to one of saidcontrol frequencies and the relay of said other circuit portion isoperated in response to the other of said control frequencies.

, 4. In a control or signal device responsive to at least two controlfrequencies to cause at least two functional operations, two circuitportions, each portion having a source of power supply, a gridcontrolledelectronic tube adapted to pass plate current from said power supply orto substantially prevent current flow, depending on the potentialapplied to a control grid thereof, a relay operatively controlled bysaid current, filter means for separating a potential drop at one ofsaid control frequencies, a separate amplifier tube connected to saidfilter means for amplifying said separated potential, second filtermeans connected to said amplifier for obtaining said amplified potentialfrom the amplifier output current, a rectifier connected to saidamplifier for obtaining a rectified potential due to said amplifiedpotential, means for filtering and delaying said rectified potential,and means for impressing a negative portion of said delayed rectifiedpotential on the grid of said electronic tube in another of said circuitportions, the filter means in each of said circuit portions beingadapted to separate potential at a different control frequency, so thatin response to one control frequency flow of the plate current in one ofsaid circuit portions is substantially prevented and in response toanother control frequency the flow of plate current in the other of saidcircuit portions is substantially prevented, whereby the relay of one ofsaid circuit portions is operated in response to one of said controlfrequencies and the relay of said other circuit portion is operated inresponse to the other of said control frequencies.

5. A control or signal device adapted to be connected to an audio outputportion of a radio receiver and responsive to a plurality of controlfrequencies to cause a plurality of functional operations, comprising aplurality of circuit portions, each portion having connecting means forconnection to a suitable power supply, a gridcontrolled electronic tubeconnected to said connecting means and adapted to pass plate currentfrom said power supply or to substantially prevent current fiow,depending on the potential applied to a control grid thereof, a relayoperatively controlled by a change in said plate current, a circuittuned to one of said control frequencies and connected through aresistance to an output portion of said receiver to obtain a potentialdrop at one of said control frequencies, an amplifier tube to amplifysaid potential drop, means for suitably connecting said tuned circuit tothe grid of said amplifier, a second circuit tuned to substantially thesame control frequency connected in series with the plate supply and theplate of said amplifier tube to obtain an amplified potential drop dueto said amplified control frequency, a rectifier to obtain a rectifiedcurrent due to said amplified potential drop, means for suitablyconnecting said second tuned circuit to said rectifier, and means forapplying a negative potential due to said rectified current to saidcontrol grid of said electronic tube in another of said circuitportions, said tuned circuit in different circuit portions being tunedto different control frequencies, so that in response to one controlfrequency flow of plate current in one of said circuit portions issubstantially prevented and in response to another control frequency theflow of plate current in the other of said circuit portions issubstantially prevented, whereby said relays in said circuit portionsare selectively operated in response to said control frequencies.

6. In a control or signal device responsive to a control frequency withwhich a carrier is modulated to cause a functional operation, a receivertuned to said carrier, a network connected to the output of saidreceiver and tuned substantially to said control frequency forseparating a higher potential drop at said control frequency than atother frequencies an amplifier tube suitably connected to said networkfor amplifying said separated potential drop, a second network alsotuned to substantially said control frequency and connected to theoutput of said amplifier for separating an amplified potential drop atsaid control frequency, a rectifyin means for obtaining a rectifiedcurrent from said amplified and separated potential, means connected tosaid rectifying means for filtering and delaying build-up of potentialfrom said rectified current, means for applying a portion of the delayedand rectified potential to a grid of an electronic tube which isconnected to a suitable power supply to modify the plate currentthereof, from said power supply, and a relay connected to the plate ofsaid electronic tube to be controlled by said modification of said platecurrent, whereby the relay in said plate circuit is operated in responseto said control frequency.

7. In a control or signal device responsive to a plurality of relativelylow control frequencies with which a relatively high frequency carrierwave is modulated to accomplish a plurality of functional operations, areceiver tuned to said carrier, a circuit portion for each of saidcontrol frequencies connected into the output of said receiver, eachportion comprising a connection to a source of power supply, agrid-controlled electronic tube adapted to normally pass plate currentfrom said power supply but being adapted to prevent said plate currentwhen said grid is made sufliciently negative, a network tunedsubstantially to one of said control frequencies for presenting a muchhigher potential drop at said control frequencies than at otherfrequencies, means for amplifyin said potential drop due to one of saidcontrol frequencies and separated by said network, a second networkdisposed in the output of said amplifying means and tuned tosubstantially the same frequency as said first-mentioned network forobtaining potential drop due to the amplified control frequency to whichsaid networks are tuned, means for obtaining a rectified current fromsaid amplified potential drop, means for suitably filtering saidrectified current and means for delaying the build-up of a negativepotential therefrom, means for applying said negative potential to saidgrid of said electronic tube in another of said circuit portions, whichother circuit portion contains a network tuned substantially to anotherof said control frequencies, so that in response to one controlfrequency a negative charge is applied to the grid of said electronictube in said other of said circuit portions to cause a decrease in platecurrent flow therethrough, and a relay suitably connected to the platecircuit of said last-mentioned electronic tube to operate by cessationof said plate current in response to one of said control frequencies.

GERALD G. ROBERTS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number

